The present invention relates to a method of preserving a polymer electrolyte fuel cell stack and a preservation assembly of the polymer electrolyte fuel cell stack. Particularly, the present invention relates to a method of preserving a polymer electrolyte fuel cell stack in an uninstalled state, and a preservation assembly of the polymer electrolyte fuel cell stack.
In the conventional method of stopping a polymer electrolyte fuel cell (hereinafter referred to as PEFC), supply of an oxidizing agent and a reducing agent is stopped and these materials remaining within the PEFC stack are purged by, for example, an inert gas such as nitrogen (see document 1). Thereafter, during a stopped state of the PEFC, an oxidizing agent passage and a reducing agent passage are typically filled with the inert gas or the like to inhibit entry of air into the PEFC stack (see document 2). Thereby, oxidization of an electrode catalyst layer within the PEFC stack and degradation of performance of the PEFC are inhibited. In addition, there has been disclosed a method of preserving the PEFC while maintaining a potential of a separator at a predetermined value during a power generation stopped state of the PEFC (see document 3).
When a membrane-electrode-assembly (MEA) is created, an electrode catalyst layer is formed by applying a coating material for formation of a catalyst layer onto a surface of a polymer electrolyte fuel cell membrane. The coating material for formation of the catalyst layer contains an alcoholic component as a solvent. As catalyst powder, for example, carbon powder carrying platinum-ruthenium alloy particles or platinum particles is used. The catalyst powder is mixed with an ethyl alcohol dispersion containing perfluorocarbonsulfonic acid polymers and produced into a paste. The paste is applied to a surface of the polymer electrolyte membrane to form the electrode catalyst layer. The solvent containing the alcohol component enters a part of a porous electrode catalyst layer and remains there after manufacturing the MEA.
As a method of improving a drawback that an ion resistance at an interface between the polymer electrolyte membrane and the electrode catalyst layer increases, and a drawback that an electron resistance at an interface between the electrode catalyst layer and a diffusion electrode layer increases because the electrode catalyst layer and the diffusion electrode layer are not firmly joined to each other, there has been disclosed a method of heating, pressurizing and integrating an element including a polymer electrolyte membrane sandwiched between two electrodes in a solvent (see e.g., document 4). Furthermore, there has been disclosed a method of heating and pressurizing a polymer electrolyte membrane and/or an electrode catalyst layer containing a solvent substantially without being immersed in the solvent (see e.g., document 5). In accordance with this method, because the solvent within a MEA vaporizes during a step of integration, swelling of the polymer electrolyte membrane that is due to the solvent is controlled, maintaining a desired joint state at the interface between the polymer electrolyte membrane and the catalyst layer.    [Document 1] Japanese Laid-Open Patent Application Publication No. Hei. 6-251788.    [Document 2] Japanese Laid-Open Patent Application Publication No. Hei. 7-272738.    [Document 3] Japanese Laid-Open Patent Application Publication No. 5-258762.    [Document 4] Japanese Laid-Open Patent Application Publication No. Hei. 3-208262.    [Document 5] Japanese Laid-Open Patent Application Publication No. 2002-93424.
As illustrated in the documents 1 through 3, it is known that an electrode catalyst layer is oxidized and thereby its performance degrades if it contacts an oxide such as air under the condition in which the electrode catalyst layer has reached a catalyst activation temperature after start of power generation.
However, inventors discovered that degradation of performance such as decrease of an output voltage, in particular, a noticeable increase in a voltage drop rate of the output voltage occurs if a long time period elapses before power generation starts after a PEFC stack is assembled. From studies conducted by the inventors, it has been found that an oxide of a solvent remaining on an electrode catalyst is a cause of the degradation of the performance.
Traditionally, little attention has been paid to the degradation of the performance before the start of the power generation of the PEFC among those skilled in the art. For this reason, the conventional stopping method of the PEFC addresses a condition after a service starts and is not intended to inhibit degradation of the electrode catalyst layer before a PEFC stack is incorporated into a PEFC system after manufactured. In other words, the conventional method of inhibiting degradation of performance of the PEFC is intended to inhibit degradation of performance of the PEFC. To be specific, since an inert gas or the like is supplied from an external peripheral device through pipes or the like connected to the oxidizing agent passage and the reducing agent passage of the PEFC stack, the conventional method is not applicable to the PEFC stack before assembled into the PEFC system, in particular, in a single piece state during storage or transportation.